If strain rate dependent deformations of metal plates are studied, it is necessary to use viscoplastic laws for the simulations of the structural response. For this purpose an efficient viscoplastic model is searched in this study.
Theoretical frameworkIn order to calculate deformations of shock wave loaded plates the viscoplastic models of Lemaitre-Chaboche [3], Tanimura [6] and Bodner-Partom [1] are applied assuming perfectly plastic material behaviour. As long as the constitutive equations of Chaboche and Tanimura are used the elastic part of material behaviour is covered by Hooke's law while the Bodner-Partom law does not seperate between elastic and plastic material properties. The Lemaitre-Chaboche model is expressed by the following equations:Here, ε p ij , σ ij , k, J 2 (), σ v denote the plastic strain tensor, the stress tensor, the yield limit, the second invariant and the overstress, respectively. The material parameters v, K must be identified from uniaxial tension test. The Tanimura model is presented as followṡwith the material parameters S 0 , r, τ * and the Bodner-Partom model is expressed bẏwith the material paramters are R 0 , D 0 , n. In order to identify the material parameters uniaxial tension tests at different strain rates are used. The identification procedure is described in [5]. For the structural modeling and numerical simulation a geometrical nonlinear first-order shear deformation shell theory is used, assuming small strains and moderate rotations. The derivation of this shell theory can be found in [4]. In order to trace the evolution of the inelastic material properties in different points in the shell space seperately, the shell is divided into layers. Details of the numerical approximation are presented in [2].
Experimental set-upFor the exerimental study two different shock tubes are used. In Fig. 1 the principle of a shock tube is shown. The tube consists of a high pressure chamber (HPC) and a low pressure chamber (LPC) seperated by a membrane. The HPC is filled with gas until the membrane bursts. Then, a shock wave travels into the LPC, impinging on the plate specimen and causing viscoplastic vibrations. The middle point displacement of the specimen is measured by a capacitor and the pressure acting on the plate is measured by pressure devices with piezoelectric elements located in a seperate ring flange next to the plate. Both quantitites, pressure and displacement, are measured during the impuls duration. Aluminium plates with 500mm diameter are used in the large shock tube, copper and steel plates with 138mm diameter are examined in the small shock tube. All plates are 2mm thick.